USC helps discover gravitational waves with Pegasus

A fleeting sound caused by the gravitational waves from merging black holes has proven that collisions create a ripple in space-time across a billion light-years of distance. USC’s Information Sciences Institute played a crucial role in the discovery that was announced on Feb. 11 by creating a program called Pegasus that helped more than a thousand researchers from the Laser Interferometer Gravitational-Wave Observatory at Caltech organize their data.

“We all feel immensely proud,” said Karan Vahi, the lead developer of Pegasus. “[We’re] glad to see all the years spent getting the pipelines up and working through Pegasus come to fruition.”

Ewa Deelman, a research associate professor in the computer science department, designed Pegasus to help more than 1,000 researchers working on the project to collaborate more effectively. Pegasus is a scientific workflow management system, providing researchers a program to work on multi-computational tasks such as retrieving data from an instrument or a database, reformatting data and running analyses.

“It was 2001 … the goal was to develop a ‘Virtual Data Grid’ for LIGO and other science applications,” Deelman said. “In 2011, LIGO passed the ‘blind injection’ test, using Pegasus. This test proved that the analysis being developed by LIGO could detect a gravitational wave.”

The process of discovering gravitational waves involved using two interferometers which utilize lasers and mirrors to measure displacements in electromagnetic waves — one at LIGO and the other with the Virgo Collaboration near Pisa, Italy. Pegasus helped analyze and sort the noise coming from the interferometers; then, once the noises were detected, it provided a way to measure whether they were significant enough to consider for deeper analysis.

“LIGO and Virgo had written computer codes to produce automated alerts and send emails for interesting candidates,” Vahi said. “The crew that was watching the alerts … reacted fast to start the needed actions to confirm it. As part of this validation process, LIGO/Virgo scientists used [an] analysis pipeline to measure the statistical significance of the detection.”

Gravitational waves from two merging black holes, neutron stars or other massive objects would produce a chirping sound. The small chirping noises may not seem like much, but to put their magnitude into perspective, supernovae or gamma ray bursts (like solar flares) produce crackling noises. The oldest gravitational waves, such as those from the Big Bang, are significantly less loud in comparison.

Pegasus has been involved with LIGO since 2001, when both groups became part of a National Science Foundation-funded project to explore the use of workflow technologies to run large-scale science pipelines on computational grids. In 2010, LIGO and Virgo decided to collaborate on developing common analysis pipelines to analyze data coming out of their respective detectors, a process that led to February’s discovery.

According to Vahi, the affirmation of black holes by LIGO has been one of the most challenging applications for Pegasus in terms of complexity, the size of data, number of files required and the number of jobs run to do analysis.

“The most rewarding part of the process has been to see techniques developed put in production use by real users,” Vahi said. “Pegasus started as a research project and its use by LIGO and other applications have enabled us transform it into a production-quality software that scientists can rely on.”